A surface light-emitting thyristors has been disclosed in the Japanese Patent Publication No. 2-14584, and an end-surface light-emitting thyristor in the Japanese Patent Publication No. 9-85985. The fundamental structure of a surface light-emitting are substantially the same, and AlGaAs (Al composition is 0.35, for example) layers are epitaxially grown on a GaAs buffer layer formed on a GaAs substrate, for example.
FIG. 1 is a schematic cross-sectional view depicting a fundamental structure of a light-emitting thyristor. As shown in FIG. 1, on a p-type GaAs substrate 10 successively stacked are a p-type GaAs buffer layer 12, a p-type AlGaAs layer 14, an n-type AlGaAs layer 16, a p-type AlGaAs layer 18, and an n-type AlGaAs layer 20. On the AlGaAs layer 20 provided is a cathode electode 22, and on the AlGaAs layer 18 a gate electrode 24. An anode electrode 26 is provided on the bottom surface of the GaAs substrate 10.
In this example, a p-type layer, an n-type layer, a p-type layer, and an n-type layer are stacked in this order on a p-type GaAs substrate via a buffer layer. However, an n-type layer, an p-type layer, an n-type layer, and a p-type layer may be stacked in this order on an n-type GaAs substrate via a buffer layer, in this case the uppermost electrode is an anode one, and the bottommost electrode is a cathode one.
In the above-described publications, the inventors of this application have already disclosed a self-scanning light-emitting device structured by arranging such light-emitting thyristors in an array, a self-scanning function thereof being implemented by providing a suitable interaction between neighbored thyristors in the array. The publications have further disclosed that such self-scanning light-emitting device has a simple and compact structure for a light source of a printer, and has smaller arranging pitch of thyristors in the array.
In the light-emitting thyristor having such structure described-above, Al composition is largely varied, for example from 0 to 0.35, at the interface between the GaAs buffer layer and the AlGaAs layer on the buffer layer. Such rapid variation of Al composition causes the turbulence of lattices or the large variation of energy bands at the interface, while the variation of lattice constants is small. As a result, a lattice-mismatching at the interface become large, thereby causing a dislocation. Also, an energy gap at the interface is increased, so that the deformation of energy bands is made large.
Therefore, for the light-emitting thyristor fabricated by growing the AlGaAs layer on the GaAs substrate interposing the GaAs buffer layer therebetween, there are problems such that a device property is degraded due to the increase of a threshold current and a holding current. This is because lattice deffects due to a lattice-mismatching at the interface between the GaAs buffer layer and the AlGaAs layer are induced, and an unclear impurity level is formed at the interface. There are also problems such that an external quantum efficiency is decreased, resulting in the reduction of the amount of emitted light. This is because defects which serve as “carrier killers” are generated in the vicinity of the interface.
As shown in FIG. 2 wherein like elements are indicated by like reference numerals used in FIG. 1, an n-type GaAs layer 28 may be provided on an n-type AlGaAs layer 20 in a conventional light-emitting thyristor. In this manner, GaAs is used as the material of the uppermost layer for the facility of making ohmic contact with an electrode and the simplicity of material. Since the wave length of emitted light is about 780 nm, the light is absorbed during passing through the uppermost layer (GaAs layer) 28 so that the amount of light to be emitted is decreased.
In order to reduce the light absorption by the GaAs layer 28, the thickness of the layer is needed to be thinner. However, if the layer is thinner, additional problems are caused. That is, alloying of electrode material and GaAs by a heat processing is required to from an ohmic electrode, and atoms migrate for a long distance during the heat processing, as a result of which the alloyed area of electrode material is reached to the AlGaAs layer 20 under the GaAs layer 28. This causes the turbulence of crystalline of AlGaAs, resulting in the scattering of light.
FIG. 3 is a graph showing a light absorption spectrum of an n-type GaAs layer at 297K, wherein ordinate designates an absorption coefficient α and abscissa a photon energy. The amount of absorbed light is represented by the following formula.1−e−αt (t; film thickness)
It is noted from this graph that the absorption coefficient for the light of 780 nm wave length is about 1.5×104. Assuming that the film thickness “t” is 0.02 μm, it is understood that the amount of emitted light is decreased by 3–4% by calculating the amount of absorbed light based on the above formula. The amount of absorbed light will be further reduced, if the turbulence of atomic arrangement is caused due to the fluctuation of film thickness and the alloying, and the variation of composition.
FIG. 4 shows a light-emitting thyristor in which a GaAs buffer layer 12 is provided on a GaAs substrate 10, and a GaAs layer 28 is used as a topmost layer. In the figure, like element are indicated by like reference numerals used in FIGS. 1 and 2.
In general, a light-emitting thyristor having a pnpn structure is considered to be the combination of a pnp transistor 44 on the substrate side and an npn transistor 46 on the opposite side to the substrate, as shown in FIG. 5. An anode corresponds to an emitter of the pnp transistor 44, a cathode an emitter of the pnp transistor 46, and a gate a base of the pnp transistor 46, respectively. The holding current of the thyristor is determined by the combination of current amplification factors of respective transistors 44 and 46. In order to decrease the holding current, it is required to increase current amplifying factors α of respective transistors. A current amplifying factor α is given by the multiplication of an emitter injection efficiency γ, a transport efficiency β, a collector junction avalanche multiplication factor M, and a specific collector efficiency α*. In order to increase an emitter injection efficiency γ, the impurity concentration of the emitter is designed to be higher that of the base.
The diffusion speed of Zn which is a p-type impurity is very fast, so that Zn is diffused into an n-type semiconductor layer to compensate an n-type impurity. Therefore, if Zn concentration of the anode layer (the GaAs layer 12 and the AlGaAs layer 14) is higher than Si impurity concentration of the n-type gate layer (the AlGaAs layer 16), then most of Si in the vicinity of the interface between the anode layer and the gate layer is compensated to decrease a transport efficiency β of the transistor. Also, non-luminescent center is generated, causing the reduction of the luminous efficiency of the thyristor.